NL9420041A - Apparatus and method for measuring the density of a formation. - Google Patents

Description

Apparatus and method for measuring the density of a formation

The present invention relates to an apparatus and methods for measuring radioactive emissions in a borehole environment. More particularly, the present invention relates to a downhole instrument and a method for improving the accuracy of such measurements, and in particular to measuring the density of formations.

Density measurements of a formation are commonly used to calculate the porosity of the formation. Using the porosity of the formation and other measured values, e.g. the resistivity of the formation, an oil or gas well can be evaluated. Furthermore, the porosity information regarding a reservoir allows the estimation of other useful determinations, such as the number of barrels of oil to be recovered. With such information, the oil extraction operator can make accurate decisions regarding the development or production of the reservoir.

Density recording is based on the detection of attenuated gamma rays emitted from a radioactive source. After the gamma rays penetrate from the source into the borehole and the formation, a fraction of the scattered gamma rays are counted using gamma radiation detectors. The scattering experienced by the gamma rays after emission from the source and before detection is related to the mass density of the formation. More specifically, the number of gamma rays so scattered has an exponential relationship to the electron density of the formation. Since the nuclear emission from a radioactive source is a random but probabilistic event, the average number of counts must be taken over a period of time long enough to obtain a number of counts, which is sufficient for a statistically accurate measurement of the number of counts.

Density measurements of a formation made during wire recording operations by drawing a density device through a borehole through an electrical wire have been available for decades. In these operations, a density device, which contains a source of radioactive gamma rays and usually two gamma radiation detectors, can be centered in the borehole so that the detectors are in direct contact with the edge of the borehole. If the detectors are not in contact with the wall, the drilling mud has a strong disturbing influence on the measurement. Usually, an auxiliary arm or spring exerts a decentering force on the device for this purpose. In order to obtain an accurate measurement, the decentralized recording device is preferably operated at a speed low enough to compensate for the counting statistics, e.g. 0.15 m / s drawn through the borehole.

Despite the decentralizing force, contact between the device and the borehole wall can be broken by a mud cake, which is often built up on a permeable formation. To correct for this frequently occurring situation, the count measurements from the detector closest to the source {the short-range detector) and the farthest detector (the long-range detector) are combined, so that a more accurate value is obtained. For this, a core and rib chart can be used, plotting long-distance and short-distance counting pulses against each other for different calibration materials and for different distances between the detectors and the formation.

More recently, measurement-in-drilling (MWD) devices have been used to measure the density of formations. The electronics of the density device and the gamma detectors (both the short-range and the long-range detector) may be mounted in a stabilizing blade, which is attached to a heavy rod in the lower part of the drill string nearby of the drill chuck. The stabilizing blade displaces drilling mud in the annulus of the borehole and brings low density windows, radially outward from the radiation source and detectors installed, into contact with the earth formation. During rotary drilling, the MWD device normally rotates at a speed of one or two revolutions per second. To compensate for the statistic, the measurement times of the data at the MWD device are longer than those at the wire density device, and are usually in the range of about 30 seconds.

The contact between the stabilizing blade and the wall of the borehole may be lost during drilling. If the borehole stabilizer blades are the same diameter as the wellbore, it can be assumed that contact with the wall is constant over the 30 second measurement period. However, it is known that the drill holes are often considerably larger than the size of the head and that this enlargement can take place simultaneously with or very shortly after the drill head has passed. This loss of contact affects the density measurement, so that the apparent density detected is greater or less than the actual density depending on the relative densities of the borehole fluid and the formation.

If an enlargement of the borehole has taken place when the hole is measured with the MWD device, it can generally be expected that a measurement lasting 30 seconds will provide data of all possible distances from the stabilizing blade to the wall of the borehole. This introduces errors into the conventional compensation technique, comparing the calculated density response of the short and long range detectors. During the measurement period, the number of counting pulses is accumulated in a linear fashion for the different borehole distances encountered during the measurement. However, the response of the device to the distance from the borehole wall to the sensors of the device is logarithmic. Therefore, the compensated response of the device to the borehole enlargement will contain a progressive error as the borehole size increases. Several methods have been developed to treat these problems.

In U.S. U.S. Patent 5-017-778, issued to P.D. Wraight, a method and apparatus is described for determining the average of successive measurements, which are preferably taken twice as fast as the speed of drilling, as well as the standard deviation of the successive measurements. These calculations are combined to provide outputs according to the variations in the borehole cross-sectional configuration and to provide indications representative of the desired formation characteristics as well as the borehole configuration. This method assumes the theoretical relationship between the mean and the standard deviation under conditions where there is constant contact between the device and the borehole wall. Under these conditions, the theoretical value of the standard deviation is almost the same as the measured standard deviation. However, if the hole has a large diameter so that the contact between the device and the borehole wall varies, the relationship between the distance of the device and the number of count pulses causes a divergence between the measured standard deviation and its theoretical value. A correction is made to the average number of counts, based on the difference between the measured and theoretical standard deviations.

Since the correction applied to the mean is derived from the standard deviation of the successive measurements, the accuracy of the method depends on the symmetry of the actual distribution of the samples around the mean. The accuracy of the correction decreases to such an extent that the actual distribution is asymmetrically around the mean. There are several factors that cause the distribution of the number of counts to be asymmetrical around the mean. For example, for stable, dynamic situations, it is not uncommon for these to be set up for a wide range of speed and weight combinations on the head, with the axis of the device moving. This movement is often in the form of a repeating pattern in the hole and can significantly affect the distribution of successive measurements around the mean. In such a case, depending on the type and degree of "spinning" or movement of the shaft of the device itself, the entire perimeter of the borehole may not even be scanned once during the entire measurement period, such as required by the Wraight method to ensure that meaningful output data is obtained It is currently difficult to control these situations in real time without the presence of additional sensors down the hole, since their occurrence need not be detectable by by means of surface measurements.

Another factor that can cause an asymmetric distribution of the data is the fact that the effect of the density on the number of gamma rays counted is extremely non-linear. For example, if the device remains on the "low" side of an elliptical hole, the device will be in contact with the borehole wall longer than if the device remains on the "low" side of a round hole. linear effect on the number of counting pulses, the distribution of the successive measurements will be asymmetrical.

In U.S. U.S. Patent 5,091,644, issued to D.C.Minette, describes a method of analyzing data from a measurement-during-drilling device for evaluating a formation to compensate for rotation of the recording device. The received signal is subdivided, preferably into four parts. As the device rotates, the detectors quickly pass through these four quadrants. Each time they cross a boundary, the counter is incremented and points to the next quadrant. Thus, the data is divided into four spectra, each obtained over a quarter of the total assay time. To determine the sector in which the device operates, the output of an additional sensor, such as an inclinometer or a magnetometer, is used. Minette also mentions that an acoustic borehole scanner can be used to divide the borehole into these sections (e.g., quadrants) based on the distance in those sections.

If the device is centered in a perfectly round hole, the distance encountered by the device will be the same in each sector and the number of counting pulses accumulated in each sector will be the same. However, if the axis of the device is not aligned with the axis of the borehole, this will not be the case and the spacing will be different for different sectors. Thus, the accumulated number of counts in each sector will be different, with the number of counts in the sector or sectors corresponding to the minimum value of the distance being the best quality for determining the density of the formation.

As with Wraight's description, in Minette's technique there is an implicit assumption that the axis of the device maintains a fixed orientation in the hole during measurement. However, there is no simple and reliable mechanism for holding the shaft of the device in a fixed location in the borehole. Thus, it is very likely that movement of the axis of the device around the borehole will occur in a vertical or near vertical hole. The possibility of such a movement never completely disappears, even in a very different hole. Furthermore, due to the location of the density sensors in the blade portion of the device, there is an increased probability of such movement of the axis of the device as the blade contacts the borehole wall.

Since the sectors or quadrants are believed to be fixed in the hole, the axial movement or translation results in a less than optimal correlation, or possibly no correlation at all, between the sectors expected to be consistent and be on a state of affairs. nucwci uc of an acoustic scanning signal, to divide the signal into bins, based on an average distance, can mitigate the problem to some extent, there is still the problem of storing the data for the correct quadrant, when assumed that the sectors arrive regularly and in succession. During the movement of the axis of the device, the same position of the device associated with the sectors and quadrants will not necessarily occur regularly and sequentially. Thus, if no sequential data storage can be canceled, the data may be asymmetrical if the axis of the device is not fixed, even if an acoustic sensor sorting signal is used.

In another method, which relates to wire recording with a single detector density device, U.S. Pat. U.S. Patent 3,321,627 issued to C.W. Tittle, a collated source and detector arrangement for a single detector density device, has been described. The concept of collimation described in this patent prevents the measurement from being affected by borehole fluids by collimating the source and detector such that the gamma rays are more likely to be directed into the formation. The device has a source collimator for directing a small beam of fixed angle gamma rays into the material whose density is determined. The device also includes a detector collimator for limiting the accessibility of gamma rays to the gamma ray detector to those gamma rays that are scattered and traverse within a narrow fixed angle, which within the formation is the small radius of fixed angle gamma rays. crosses from the source. A 1985 article by Hearst and Nelson entitled Well Logging for Physical Properties discusses the related concepts of density measurements, and in particular single scatter density measurements.

However, there remains a need for an improved methods and apparatus for more accurately measuring the radiation in a wellbore environment, addressing the problems encountered with prior art devices making such measurements, including poor accuracy measurements taken at different distances from the borehole wall are overcome. Experts have long sought and will appreciate this invention, which provides solutions that substantially alleviate these and other problems.

The present invention provides a method of determining at least one feature of an earth formation that is penetrated through a longitudinal axis borehole. A first directional radiation sensor is placed in the borehole next to the earth formation, to detect the number of count pulses indicative of the radiation received from the earth formation. The detector may be of a type sensitive to different types of radiation or emissions, such as gamma rays and / or neutrons, and to different energy ranges of that radiation or emissions. The first directional radiation sensor is rotated at a rotational speed with a single rotation period and the number of counts detected is stored during the rotation for a total measurement period. The term "single rotation period" as used herein does not mean that the drill string speed is constant, but refers to the rotation period for a rotation of the downhole device. However, this single or one rotation can be considered constant with respect to the sampling period during that one rotation of the device. The total sampling period is longer than twice the single rotation time. The step of storing the number of counting pulses includes storing the number of counting pulses for each series of short measurements performed during the entire sampling period. Each of the short measurements are taken for a short time segment. The short time segment is less than half and preferably less than a quarter of the single rotation duration. A plurality of bins is defined as a function of at least a portion of the distribution of the number of counts of the counts detected during each short period. The stored number of counts is preferably retrieved and an average number of counts of the total sampling period is determined from the number of counts detected during a series of short measurements. Each of the short measurements is then sorted into one of preferably at least three bins, each bin having limits defined as a function of the average value of the number of counts, to provide an indication that is accurately representative of the characteristic of the earth formation in which is measured.

The method preferably includes positioning a radiation source in the borehole and positioning a first directional sensor axially closer to the radiation source than a second directional radiation sensor. The radiation detected by the first directional sensor is blocked so that radiation from the earth formation, from a radial direction substantially perpendicular to the longitudinal axis of the borehole, is substantially prevented from being received by the first directional sensor.

The apparatus of the present invention includes a short range detector and a long range detector for examining the properties of earth formations around a borehole irradiated with radiation from a radiation source. The device preferably includes a generally round body with a longitudinal axis that is substantially co-axial to the borehole. A short-range detector collimator is mounted at a first distance from the radiation source in the round body and is filled with a material which is substantially transparent to the radiation. The short-range detector collimator has a short-distance cross-section, which is delimited by a plane crossing the collimator, and is parallel to and contains the longitudinal axis, with a first side of the short-distance cross-section, which is furthest from the radiation source, is oriented so that a first acute angle to the longitudinal axis is formed, which allows the radiation passing through the detector for the short distance from a direction that is substantially perpendicular to the longitudinal axis of the device is received, is limited. A long-range detector collimator is mounted a second distance from the radiation source in the round body. The long-range detector collimator is made essentially of a material that is substantially transparent to the radiation. The long range detector collimator has a wall extending radially outward from the long range detector to transmit radiation in a direction substantially perpendicular to the longitudinal axis of the device.

The method includes providing an irregular borehole flag to determine when to make corrections for an irregular hole size. To do this, the stored number of counts is recalled, and the average of the total number of counts and the measured standard deviation of the number of counts are determined for the number of counts detected during each series of short measurements. The standard deviation of the total number of counted pulses is compared to the theoretical standard deviation, which is calculated from the average of the total number of counts, and an irregular borehole flag signal is produced as the standard deviation of the total number of counted pulses deviates more than specified from the calculated standard deviation. However, the correction itself is not determined by the measured standard deviation.

Another embodiment of the present invention provides, before positioning, a downhole acoustic borehole sensor, aligned with the first directional radiation sensor, for receiving acoustic signals functionally related to the distance from the first directional radiation sensor earthing in the borehole. A series of short measurements are taken and each of these measurements is sorted into at least two bins. The bin boundaries are determined by the distance detected by the scanner, regardless of the order in which the series of measurements was performed. Data in at least one of the multiple bins is kept.

The method of the present invention preferably includes forming the short-range collimator to pass a portion of the gamma rays to the short-range detector such that a first azimuthal width of a portion of the short-range collimator distance is less than about the diameter of the housing for the device. The long-range collimator is preferably constructed with an azimuthal width greater than at least three times the first azimuthal width of the portion of the short-range collimator.

An object of the present invention is to provide an improved method of determining the porosity by correcting the number of radiation count pulses detected by borehole devices.

Another object of the present invention is to increase the distance range for which the corrected density is relatively accurate.

A feature of the present invention is a method of sorting the number of detected counts during short measurements based on the average of the short measurements over a much longer sampling time.

Another feature of the present invention is a detector collimator for obtaining more accurate correction information for the core and rib for formation density measuring devices.

An advantage of the present invention is the fact that the method of correcting the information of the number of radiation count pulses is effective regardless of the movement of the axis of the measuring device during drilling.

Another advantage of the present invention is a method of correcting the information regarding the number of radiation count pulses operating even though the response of the formation to the detectors changes logarithmically.

Other features and intended advantages of the present invention become apparent with respect to the following detailed description and accompanying drawings.

Fig. 1 is a view of a measuring-during-drilling device according to the present invention in a borehole;

Fig. 2 is a cross-sectional view of a collimator scheme according to the present invention;

Fig. 3 is a cross-sectional view of another collimating device scheme according to the present invention;

Fig. 4 is a cross-sectional view along line 4-4 of FIG. 3;

Fig. 5 is a cross-sectional view along line 5 "5 of FIG. 3;

Fig. 6 is a cross-sectional view taken along line 6-6 of FIG. 3;

Fig. 7 is a cross-sectional view taken along line 7-7 of FIG. 3;

Fig. 8 is a core rib chart of the present invention;

Fig. 9 is another core-rib graph according to the present invention;

Fig. 10 is a graph of density correction according to the present invention.

While the present invention is described with respect to the presently preferred embodiments, it will be understood that the present invention is not limited thereto. In contrast, it is intended to encompass all alternatives, modifications and equivalents as encompassed by the spirit of the present invention and as defined in the appended claims.

The present invention provides an improved method and apparatus for determining at least one feature of an earth formation in which a borehole is drilled and more particularly includes techniques and devices capable of obtaining accurate radiation measurements, which are indicative are for the density of the formation. Density measurements, which are made by measuring-during-drilling techniques (MWD), can be corrected to provide the drill operator with more reliable information.

In the drawings, and more particularly in Figures 1-3 «, a device or apparatus 10 for measuring the density of a formation is shown. Preferably, both a radiation source and a pair of detectors are provided in blade 11 of heavy rod 13, which is usually a tubular body. Heavy rod 13 is usually screwed to drill string 15, which can be rotated so that head 17 rotates in a conventional manner and drills through an earth formation 19, forming borehole 21 with wall 23. Heavy rod 13 preferably includes two or more additional blades, such as blade 22. Heavy rod 13 may be located at various locations along drill string 15, but is preferably located near head 17. to provide measurements as close as possible to the near the portion of the borehole 21 immediately above the head.

Device 10 includes a long-range (far) detector 12, a short-range (close) detector 14 and a radiation source 16, each of which is generally shown in Figures 2 and 3. The radiation source 16 is preferably a cesium-137 type source. A long-range collimator 18, a short-range collimator 20, and a source collimator 24, as shown schematically in FIG. 1, are radial to outside of the detectors and source 12, 14 and 16, respectively, as discussed in more detail below. Parts such as shield 28, sleeve bore liner 30, and outer liner 36 may be made of a high density material, such as lead or tungsten, to prevent gamma rays from source 16 to detectors 12 or other than intended. 1 * 4 go. Shock absorbing material at 33, 35 and 37 as shown in Fig. 3, and elsewhere can be used to protect the detectors from excessive vibration. The electronics package 39, as generally shown in Fig. 2, may include a memory, a voltage supply, regulators, a transfer circuit and other components for operating the device, as explained below.

The primary density sensor in a preferred embodiment of the device 10 is a long-range detector 12. For the long-range detector 12, the number of counting pulses received from an earth formation 19 when irradiated from source 16 with gamma rays changes logarithmically. with the electron density of the earth formation 19. However, if the device 10 is not against the borehole wall 23, the distance or edge opening 32 is filled with an intervening borehole fluid 26, which receives the gamma rays received by the detector 12, and thus the density measurement. This results in an apparent density which, depending on the relative densities of the borehole fluid and the formation, is greater or less than the actual density.

Preferably, a short range detector 1 * 4 is used to provide a distance correction. With the shorter distance and other factors discussed below, greater sensitivity to the width of the aperture 32 (also called the distance from the device) is obtained than when using only the long-range detector 12. The long range detector 12 and the short range detector 1 * 4 are preferably calibrated such that they both give the same apparent density when the distance is zero, i.e. when there is no gap 32. However, as the distance increases from zero, the different responses of the two detectors to intervening borehole fluid result in different densities detected by the two detectors.

The behavior of the two detectors by distance is often represented as a core and rib graph, as shown in Figures 8 and 9. This graph mainly plots the response of the long range detector 12 against the response of the short range detector 1 * 4 for different materials, distances and densities of the borehole fluid. Fig. 8 generally depicts the core and rib lines defined by the responses of the nearest (short) and (long) detectors. Fig. 9 shows the response of the device for different materials with different distances. Although Fig. 9 shows the apparent density according to the near detector plotted with respect to the apparent density according to the far detector 12, this graph can also show the number of counts according to the near-density detector 14 versus the number of counts according to the far detector. 12. The corrected density values can be obtained in a manner known in the art, by finding a point on a rib, or an interpolated rib, defined by the response of the two detectors, and tracing the rib back to the core, which is usually calibrated with a scale (not shown), to represent the corrected density. As the distance increases, the graph of the rib eventually returns to the core, with both detectors giving the same density, which is the correct density of the borehole fluid.

Figure 10, which is derived from the core and rib charts, can usually be used to provide a correction, showing on the vertical axis the difference between the corrected or true density and the density according to the long range detector 12 (ie distance correction). On the horizontal axis, the difference in densities detected by the long-range detector 12 and the short-range detector 14 is plotted. For a preferred embodiment of the present invention, the behavior of the distance correction is such that the correction can be described by a polynomial function of one variable (the difference in the near and far apparent densities) for distances up to about 2.5 cm and for mud weights up to about 2.0 kilograms / liter.

At distances greater than 2.5 centimeters, the short-range detector 14 begins to saturate, seeing only mud (detects gamma rays that are only responses to mud). The accuracy of the detection measurement at less than 2.5 centimeters depends on various factors, such as the collimators, which improve the response of the device for distances less than 2.5 centimeters. The method of the present invention is used to provide an indication of the situation of extremely long distances, where the core-rib correction technique deteriorates, yet providing an improved method for increasing the distance range in which the corrected density is relatively is accurate. In addition, an acoustic scanner can be used for a variety of purposes, such as checking the size of the borehole before formwork and determining the volume of the borehole, and thus the amount of cement needed, to secure the formwork in place. cementing. Thus, while the device of the present invention provides a more accurate distance correction between zero and 2.5 centimeters, the method of the present invention effectively increases the distance range in which the density value can be accurately corrected.

Thus, using the indication (flag) of the present invention, it is possible to determine whether to use the correction method of the present invention, as discussed below. Preferably, core-and-rib-derived correction graphs, obtained from measurement data collected over a long period of time, are used, unless the measurement data, on the other hand, has to be corrected due to an enlargement of the borehole, which is at a distance of more than about an inch. The indication of the present invention provides a simple method of detecting when such correction is desired.

If the augmentation of the borehole 21 has occurred while measuring the apparent density of the formation 19 by the device or device 10, it can generally be expected that a measurement taken over a period of 30 seconds (usually a period of time that many rotations of the drill string 15) is performed, includes data of significantly varying distances from the stabilizing blade 11, which is spaced from the wall 23 of the borehole. This varying distance introduces an error in the core-rib based compensation schemes or other conventional schemes comparing the calculated density response of the two detectors as these schemes are based on isolated responses of the two detectors to static situations. During the sampling period, the number of counting pulses is linearly integrated or accumulated for the different distances 26 in the borehole encountered during the measurement. Thus, the response to the number of counts of device 10 for a large number of static, possibly random, measurements will be linearly combined. However, as discussed above, the response of the device 10 to the distance from the earth formation is logarithmic. Thus, without a correction other than a core-rib type correction, the error of the compensated response of device 10 will progressively increase as the size of the borehole 21 increases with respect to the external effective diameter of the device 10. Although it is possible that the device accurately corrects some degree of the extra size of the borehole, the hole can be so sized that the core sibbe correction of the mean values is no longer sufficient.

Thus, the method of the present invention uses rapid sampling to shorten the sampling period to the point where the rapid sampling is much shorter than the single rotation period of the device 10 or the drill string 15. The shorter sampling period includes data from a much wider range of distances (ie data from only a portion of the rotation of the device 10) and will consequently follow a correct rib on a core and rib plot much more accurately. However, due to the short sampling periods, the statistical noise in the data will increase significantly, and so the accuracy of the correction based on the core and rib chart will contain statistical noise. Even with the statistical noise for sampling intervals as short as 50 or 100 milliseconds, the variation in the number of counts between distance is zero and long distances (eg distances greater than about 2.5 centimeters, for which the core-rib correction no longer applies) large enough so that a distinction can be made between these situations. Since the density of the formation does not change significantly during a single rotation of the device 10, any statistically significant change in the number of counting pulses (i.e., a change greater than the usual variation may be due to the random nature of nuclear events) are attributed to the variation of the distance between the device 10 and the formation 19

For a standard hole size, the device will always be in contact with the borehole wall. In this case, the only variation in the counting pulses during a given measurement period will be due to recognized variations in the number of nuclear counting pulses, which conform to Poisson statistics. For larger sized holes, different short samples correspond to different distance values, and therefore the variation in the number of counts has an additional component, which can be assigned to the changing distance. Thus, the distribution of the number of counts for a series of samples extending over several rotations of the device 10 can be compared to the Poisson distribution expected for a standard borehole. Any statistically significant difference in the two distributions implies an enlarged borehole. It is then possible to examine the measurement data of individual short periods and those measurement periods, which are too many counts (typically mud less dense than formation) or low counts (relatively heavy mud) give a lower weight or disregard it (in which case the weight is zero). Sampling periods with an extremely low or very large number of counting pulses are those periods that produce measurement data adjacent to the correct rib. As a result, the average of the remaining rapid samples is closer to the correct rib or graph.

A total sampling period is chosen, which usually has a duration of the order of about 20 or 30 seconds, and therefore includes a period which is usually much longer than twice a single rotation of the device 10. The total sampling period of 30 seconds can thus be divided into a series of quick samples or short measurements of 50 milliseconds. These short sampling periods are preferably consecutive, each period lasting a fraction of a second. Thus, the total sampling period is usually made up of an integral number of short, consecutive sampling periods. Thus, in one embodiment of the present invention, the number of samples in a given short sampling period bin, as discussed below, can be added to determine whether the total sampling period is sufficient to sufficiently remove the statistical variations.

The duration for a quick sampling or short measurement must be short enough to obtain samples, which are located essentially at the ends of the range of distance changes, as well as distance values between these extremes. The shorter the sampling duration, the easier this criterion is met. However, the shorter the sampling duration, the greater the statistical noise will be. Fifty milliseconds is a fairly short period of time, which can be used for most conditions, although a shorter period can be used. Thus, the minimum number of rapid samples for a rotation should be about four or five, so that a high probability is provided that at least one sampling takes place very close to or against the borehole wall during each rotation of the device. 50 ms samples provide 6.7 samples per rotation of the device at 200 rpm. The short measurement, according to this invention, is less than half of the single rotation time of the device.

Preferably, the data or variables are collected in different energy windows for each detector in a manner known in the art. These may include a Pe (photoelectric energy) window for the near (short range) detector 14, denoted as PeN, a far (long range) Pe window, denoted as PeF, a near density window pN, and a density window pF. PeN and PeF are related to gamma rays received with gamma ray photoelectric energy levels. pN and pF are related to gamma rays detected with energy levels of the gamma rays in the Compton scattering region. The total number of counts (TC) is defined as the sum of the number of counts of the short-range and long-range detectors in the energy interval from 50 KeV to 450 KeV. This energy trajectory includes all or most of the four windows discussed above.

In one embodiment, short samples are collected for each of these windows and for the TC using a rapid measurement method. If the total measurement time (T) is 30 seconds, with the duration of fast or short sampling (N) 50 milliseconds (t), then T = N x t, where, in this case, N = 600 short sampling. The short samples are stored in a memory, which is preferably located in the hole, but which may be surface if the transfer rates from the device to the surface are sufficient for this. In another embodiment, complete spectra for each of the 50 ms samples are collected, and the window and statistical analysis is completed after completing N samples.

After 30 seconds, there are five vectors or series of measurements, i.e., PeN, PeF, pN, pF, and TC, each comprising 600 quick or short measurements. The average of the number of counting pulses for each short sampling of the TC vector can be calculated using Equation 1:

(1)

The theoretical standard deviation of TCGem is calculated as follows:

(2)

The actual measured standard deviation for the TC vector can then be calculated:

(3)

To determine the indication for enlarged boreholes discussed above, the ratio of oActual to oTheory is considered. This ratio gives an indicator of the size of the hole, which indicator results in an HSI indication when it reaches a certain value, which value depends on the weight of the mud.

(* 0

While the above comparisons are preferred comparisons, similar or related comparisons can also be used. The HSI is preferably stored for each 30 second long sample, and can be stored as an 8 bit number, representing values from 0 to 12.75 in 0.05 steps.

The TC vector is preferably used to sort each of the other vectors PeN, PeF, pN and pF into at least three respective bins. Other bins numbers can be used, but three bins are currently preferred. A bin is simply a classification of data and is usually a memory area or memory indicator for storage. Since bins are usually storage areas, if a group of data is known to be invalid, the data from that bin can be placed in a delete bin and deleted instead of stored.

For this, three bins for PeN are designated as PeN1, PeN2 and PeN3 · Similarly, three bins are designated for each of the other vectors, i.e., PeF1, PeF2 and PeF3. Bin 1 then includes all short samples for which the value of the TC vector for that sample is less than TCGem minus a number, such as a theoretical or measured standard deviation or another number based on a type of standard deviation. The currently preferred number for bin 1 is TCGem - (^ Actual) /2.33- Bin 2 is determined similarly, the range of currently preferred values including those for which the value of the TC vector is between TCGem (° actual) /2.33 and TCGem + (actual) / 2.33. Bin 3 includes all values of TC that are greater than TCGem + (oReal) / 2.33- It is noted that the number 2.33 divides a normal or Gaussian distribution into three equal parts. Such a consideration can be used when selecting other numbers, where a division can be divided into five parts. For example, the theoretical standard deviation can be used to divide the data into five bins.

The letter i can be used to designate short samples for any vector. In the above-described embodiment, each vector has 600 elements or values, denoted with a value of i from 1 to 600. The following sorting can be performed based on the value of i for a given short sampling. If TCi is in Bin 1, PeNj is accumulated in PeNl, PeFj is accumulated in PeFl, and so on for each vector. If TCi is in Bin 2 or Bin 3, the value of the short duration or measurement for that value of i is accumulated in PeN2 or PeN3, PeF2 or PeF3, etc.

Since the number of counts for each of these bins must be calculated, it is also necessary to add or store the number of short samples and the duration of each sample accumulated in each bin. Name these NI, N2 and N3 · So for every TCj such that 0 <TCi <TCGem - oWerk./2,33, NI is increased by 1. If TCGem - oWerk / 2.33 <TCi <TCGe „, + oWerk / 2.33, N2 is increased by 1. In the hole memory storage, previously stored count pulses can be placed in any of the four windows in any of the three bins. To obtain the number of counts for each of the bins, the number of samples ultimately accumulated in each bin is also required. If the total number of samples N is constant or otherwise, it is only necessary to store information in two of the three bins, because the number of counts stored in the third bin is the total number of counts minus the number stored in the other two bins counting pulses. In other words, this means that NI + N2 + N3 = N always applies. Such additional information can be stored for control purposes if desired.

Assuming that the density of the formation is greater than the density of the mud, the density of the formation, in the presently preferred embodiment of the present invention, can be calculated by averaging the number of counts for the respective windows or vectors corresponding to Bin 1 or Bin 2. Next, the density is corrected using a core-rib chart type correction. It is also possible to determine the density based on the number of counting pulses in a single bin, such as Bin 1, since the measurements of bin 1 were probably taken at the shortest distance. Also, a weighting system for the bins can be used, perhaps based on the difference in densities determined for each bin instead of omitting an entire bin of information (or zeroing it). If the density of the formation is less than the density of the mud, Bin 3 should be replaced with Bin 3 in the discussion above.

Although in the presently preferred embodiment, the average of the number of counting pulses is used to define the bins, other parts of the division of the counting pulse may be used. Basically, the bins are defined in terms of the distribution of the number of counts. The means and some form of standard deviation of the distribution of the number of counts is currently the preferred method for defining the bins. Other segments or parts of the distribution of the number of counting pulses can be used. For example, only those measurements in a portion of the count-pulse distribution with a number of counts, indicating that they were taken near the formation, can be used. This may be a percentage of the distribution that differs from the presently preferred distribution, which is based mainly on the average of the number of count pulses discussed above.

To conserve computer memory, one can choose not to accumulate Pe correction data, or the Pe can be calculated using only one detector, preferably the remote detector. It is possible that the software in the hole determines when the series of short measurements based on the HSI indication discussed above is processed. With a sufficiently small value of the HSI, the sorting process can be avoided, reducing computer time and saving memory.

Another sorting method involves the use of an acoustic scanner. Items 62, 64, 66, 68 in Figure 2 diagrammatically represent an acoustic velocity device, with transmitter 62, receivers 64 and 66, and electronics package 68 being capable of providing information from the acoustic scanner. In this case, it may be desirable to sort the vectors in bins, as described above. However, the accumulation in these bins in this case can be performed while taking the data from the short samples, rather than after retrieving the data from the memory. Placement in the bins is not limited to the order in which the data is obtained during the short sampling.

In the simplest embodiment of an acoustic distance measurement, the signal obtained from the acoustic device is an indication of the back and forth propagation time of an acoustic pulse between the device 10 and the wall 23 of the borehole. Other relevant data, such as the acoustic velocity in mud, the resolution of the acoustic device, the diameter of the borehole and the instantaneous position of the device in the hole, can be calculated from simultaneous signals from multiple acoustic sensors. The round trip propagation time data from a single acoustic device can be divided into appropriate intervals corresponding to the relevant distance ranges. Then, the binding process can be performed based on the measured back and forth propagation time. In water, the round-trip propagation time for a distance of, for example, 0.508 centimeters (0.2 inch) is 7 x 10 6 seconds (7 microseconds). Thus, the first bin can include any data for which the acoustic time was seven microseconds or less, which corresponds to a distance of zero to 0.2 inches; the second bin can contain all data for an acoustic time between seven and fourteen microseconds, which corresponds to a distance of 0.2 to 0.4 inches, etc. The bin process of a predetermined number of samples can be done as before, or because sorting is done during measurement, the number of samples in the bins can be checked to determine if an appropriate number of fast samples is present to obtain a value that is valid for a predetermined statistical accuracy, and the long measurement period can be shortened accordingly or extended. The acoustic sensor data may also provide an indication, similarly to the HSI indication, for determining when a correction of the data is necessary.

Another preferred method of using the data in the selected bins, or all data if the hole size is too small to set the HSI indication, as discussed, effectively requires linearizing the number of counts versus the distance for both the near and far detectors. As discussed, the response of device 10 is logarithmic with respect to the distance. More specifically, the number of counting pulses from the far detector 12 varies exponentially or logarithmically (non-linearly) with distance. The response of the number of counts of the short range detector 14, as preferably collimated in a manner discussed, varies about linearly from a distance of about 2.5 mm to about 3 * 8 mm (about an inch to about one and a half inches). When both numbers of counting pulses are linearized, the mean number of close counting pulses is the same mean distance on a curve of the number of near counting pulses versus the mean distance as for the far detector. During operation, the sum of the logarithms of each measurement of the number of counting pulses at a long distance can be averaged. Then, this average, with the average number of short-distance counts, which is linear, is used to produce the corrected density.

The acoustic scanner signal, if calibrated to determine the distances, can also be used directly, rather than indirectly, for correction purposes, using the distance as part of the correction data. Also other methods and devices can be used to effectively generate the instantaneous value of the borehole spacing.

While a correction method has been discussed which increases the range of accuracy of the device as the distance becomes relatively great, it is also desirable to make accuracy adjustments for smaller spacings, ie for distances less than about 2.5 centimeters. enlarge. Thus, the method and apparatus of the present invention provides improved correction not only for long distances, but also for short distances. For this purpose, the long-range detector 12, the short-range detector 14 and the source 16 are preferably aligned and aligned along a line parallel to the axis of the device in the device 10, which axis is usually in substantially parallel to the longitudinal axis of the borehole 21. The gamma rays leave source 16, scatter through formation 19, and some gamma rays scatter backward to device 10 and are detected by detectors 12 and 14. The intensity of the radiation decreases with an increasing distance from the source. Thus, the flux available at the long-range detector 12 is less than the flux available at the short-range detector 14. Due to the greater distance from source 16, the gamma rays detected at the long-range detector 12, however, are more likely to be scattered deeper into the formation. Thus, the gamma rays detected with the short-range detector 14 are less likely to have been far from device 10 and are more sensitive to the environment near device 10 as these gamma rays are scattered less on average than the gamma rays detected by the long range detector. Therefore, the long range detector 12 of the present invention is preferably considered the primary density meter, with a borehole disturbance correction being provided by the short range detector 14. Thus, the core and rib charts, as shown in Fig. 8 and Fig. 9, characterize the response of the device 10 in the presence of interfering material, such as borehole fluid.

Although the diameter of the sensitive parts of the MWD density device 10 may correspond to the "standard" diameter of the drill bit, and although the recording with the device 10 may take place just after drilling borehole 21, at the moment of the registration almost always some enlargement of the hole has taken place. This variance may be due to the "swing" of the head into the hole, due to rotary drilling after a "bend" or due to the flushing of the hole after the head with the flow of drilling fluid. In the method and apparatus of the present invention, the correction derived for the core and rib plot is optimized by increasing the sensitivity of the long range detector 12 to the formation while simultaneously the sensitivity of the short-range detector 1 * 4 to the environment in the vicinity of the device 10, ie the borehole, is increased. To this end, the long-range detector 12 is placed as far as possible from the source, without the gain obtained by sensitizing the long-range detector 12 being offset by an increased statistical measurement error due to a reduced number of counts. The actual point at which this occurs is a function of, among other things, the source strength and the time available to obtain a reading.

The gamma rays for both the near and the near detectors 12 and 1 * 4 pass through collimators 18 and 20 into the stabilizing blade 11 of the device 10. Ideally, these collimators should be at least substantially of low material density filled, which has a low atomic number and is effectively transparent to gamma rays, compared to the material of the device around the collimation devices, such as screen 28, drill screen 30 around drill 38 and outer surface sleeve 36. To maximize the number of counts at the detectors the collimator 18 for the long-range detector 12 preferably has a cross-section approximating the cross-section of the detector 12, as shown in Fig. 3, and preferably has a longitudinal axis substantially parallel to the axis 3 ^ of the establishment. Gamma rays entering the long range detector 12 do so at an angle almost perpendicular to axis 3 **, rather than parallel to it. As previously noted, gamma rays detected by the long range detector 12 are usually scattered a number of times in the formation. Because of the orientation of the collimator 18 and the axial distance between the long range detector 12 and the source 16, each of the gamma rays detected in the far detector has a relatively high probability of being largely in the formation 19 have been scattered.

The sensitivity of the short range detector 1 * 4 to the environment of the borehole is increased by decreasing the axial distance between this detector and the source 16. However, the mechanical strength and shielding requirements provide a practical limit on the distance between the short-range detector 1 * 4 and the source 16. A radially outwardly directed collimator for the short-range detector 1 * 4 provides a useful rib from which corrections can be made, as shown in FIG. of the radial window rib 12 is shown. A more sensitive detection scheme, as provided by the device of the present invention, as discussed below, includes a rib as referred to as wedge window 70 in Fig. 8. The greater the difference in response difference between the short-range detector 1k and the long-range detector 12, the greater the accuracy of the correction, at least for relatively short distances. In other words, according to the method and apparatus of the present invention, the degree to which the detectors respond differently as a function of the distance is increased. Thus, the correction value is a function of the difference between these two values, and for a sufficiently small difference, the measurement errors in the detectors become more critical to determine an accurate correction value. Therefore, the present invention provides a relatively large response difference to provide a more accurate correction at least for relatively short distances. Compare Fig. 8 and Fig. 10. With the method of the present invention, as discussed above, this accuracy is effectively extended from smaller distances to longer distances.

Gamma rays in the formation and the borehole are scattered randomly. Some of the detected gamma rays scatter in the environment only once, others twice, others three times and so on. The probability of detecting a gamma ray scattered only once or twice decreases with increasing distance from the source. The long range detector 12 is spaced such that most of the gamma rays detected thereby are scattered several times.

A narrow collimating source 16 of gamma rays and a short range detector 1 * 4 is illustrated by the collimators 20a and 24a, as shown in Fig. 2. For illustrating the principle, even these relatively narrow collimators can be more closely presented. . Furthermore, source collimator 24a and short range detector 20a are oriented at angles smaller than perpendicular, i.e., at an acute angle to axis 34 of the device. Such an orientation, assuming that the surrounding material 38 of the device is perfectly opaque for gamma ray passage, substantially allows only gamma rays to be detected entering the bore hole 40 and returning to the short range detector 14. Multiple scattering is possible, but the probability of exiting source collimator 24a and entering short-range collimator 20a in the direction that allows passage to detector 14 after multiple scatterings is negligibly small compared to the probability of this after a scattering. Thus, the narrow collimators allow only single scattered gamma rays to be detected and, which may be more important, define the location of the scattering event.

The scattering site of the detected gamma ray can be brought closer to the device 10 by decreasing the angle of the narrow collimators and can be further removed by increasing the how. In fact, the location of a single scattering for a radially oriented collimator is extremely far from the device. For sufficiently narrow radial collimation, the single scattering site may be so far away that the probability of detecting a single scattered gamma ray is virtually negligible. For practical purposes, it is more likely to detect those gamma rays, which are more often scattered at one time and which have succeeded in reaching the detector via a more dramatic but shortcut.

The survey area in the general area of the opening 40 can be enlarged by broadening the collimation in a plane, as shown in Fig. 3, defined by the central axis 3 ^ of the device and passing through source 16 and detectors 12 and 14 so that the profile of the short-range collimator 20 and / or the source collimator 24 is wedge-shaped. As shown, the longitudinal length of the collimator 20 increases with the increasing radial distance. Thus, the area from which the gamma rays can scatter once and then be detected is broadened. To generalize the principle of the single scattering, the detector's depth of sensitivity is increased as the opening of the collimator is extended more in a direction perpendicular to the axis 34 of the device. In order to extend the area more towards the device 10, the collimator must be oriented at flatter angles. An acute angle of wall 44 of collimator 20 relative to the longitudinal axis 34 of the device prevents radially displacing gamma rays from reaching the short-range detector 14. The sharper angle of wall 42 of collimator 20 increases the number of count pulses emitted from source 16 and detected by the short-range detector 14.

In a preferred embodiment, it is desired that the collimation of source 16 be reduced until it has a wedge-shaped cross-section, as shown in Fig. 3. A narrow opening of the collimator for source 16 unnecessarily limits the array of gamma rays for single scattering events. . Regardless of the interests of the short-range detector 14, it is generally desired not to unnecessarily limit the source collimator 24 in order to maximize the number of counts for the further removed detector 12.

However, the physics of the gamma-ray scattering from electrons are more complicated, because scattering in certain directions is preferred. Scattering in forward directions, or at angles representing small deviations from the original gamma ray trajectory, are more likely than scattering at most larger angles, representing larger deviations from the original gamma ray trajectory. More specifically, those gamma rays which are scattered at a small angle, i.e., such an angle that the path of the gamma rays is more parallel to the axis 34 of the device, tend to enter the short-range collimator 20 to enter through that portion of collimator 20, which is at the greatest angle to the radial direction, ie into the opening of wall 42. Those gamma rays, which are scattered at a greater angle, tend to enter collimator 20 via that portion of the collimator, which makes the smallest angle to the radial direction, ie in the opening of wall 44. Accordingly, since the collimator is opened to progressively increased angles from the perpendicular direction, the number of events detected of roads with a small angle disproportionately. On the other hand, by increasing the "wedge" shape of collimator 20 in the plane shown in Fig. 3, to small angles to the perpendicular direction, the number of events is not increased proportionally, since the probability of scattering decreases over such larger angles.

Under certain circumstances, it may be desirable to change this sensitivity by changing the relative number of gamma rays scattered at a small angle to those scattered at larger angles. This can be done in several ways. For example, the angle of wall 42 relative to a radial direction can be decreased, excluding those gamma rays that are scattered at the smallest angles. However, this method reduces the sensitivity to small distances.

A preferred method includes widening the collimator 20 in the azimuthal direction, i.e., in a cross-sectional plane orthogonal to axis 34 of the device, as one moves farther from the source. This allows the acceptance of more gamma rays that are scattered over a larger angle. In Fig. 1 the general configuration of the short-range collimator 20 is illustrated and in Fig. 4 the cross-sectional configuration of the short-range collimator 20 is more clearly shown. The wedge shape of the short range collimator 20 should ideally narrow as the wedge approaches detector 14. In a plane through the collimator 20 and orthogonal to the axis 34 of the device, side 44 of the collimator is wider than side 42, as shown in Fig. 1. Such a shape compensates, at least in part, for loss of intensity of the gamma ray flux at larger scattering angles, since this collimator configuration accepts a greater flux of gamma rays scattered deeper.

This modification allows collimation acceptance and detection of gamma rays scattered at larger angles, but keeps the number of gamma rays scattered at small angles substantially constant. This increases the relative fraction of deeper scattered gamma rays. The deep sensitivity is ultimately reflected in the shape of the rib, such as rib 70. Such a modification, like the general wedge shape of collimator 14, as discussed above, provides greater control over the volume of the space outside the device 10 to which device 10 is sensitive.

For this, various shapes of the collimator 20 can be used, the azimuthal width of the collimator being increased with the distance from the source. For example, collimator 20, for the most part, starting at the end 42 closest to source 16, may be very narrow and, on the other hand, become suddenly and considerably wider.

Another related consideration is the azimuthal acceptance angle of the collimators. The derivation of correction data for the distance from the device is usually based on the characterization of the response of the device in a controlled situation. Thus, a difference between the calibration of the device for different distances in a ten-inch laboratory borehole may produce a different response in a borehole which, due to the difference in borehole curvature, is increased to eleven inches. The response can also be affected by the orientation of the device in the borehole.

To minimize this effect on the short range detector 14, the collimation of the near detector in the azimuthal direction should preferably be narrow, thereby minimizing this effect in the azimuthal direction of any measured round material. However, there are at least two ways to "narrow" collimation or limit a "beam" of detected gamma rays. One method, as discussed, is to decrease the azimuthal width of the collimator. Another method of effectively narrowing the collimator is to extend the path length of the gamma rays through the collimator. For example, in the presently preferred embodiment of the device having a diameter of about 24 cm (8.5 inches), the short-range collimator 20 is inclined at an angle of about 45 ° to the longitudinal axis 34 of the decor inclined. This tendency extends the path of the collimator from about 2.5 cm or an inch (if the collimator centerline was perpendicular or 90 ° to the longitudinal axis 34 of the device) to about 3.75 or one and a half inches . Accordingly, the collimator is extended about 50%. Preferably, the azimuthal width of the collimator for this length is about 0.6 cm (quarter inch) for much of the collimator, so that the azimuthal width averages this value. Thus, a preferred length to average width ratio can be in the range of about 6: 1 or greater. Increasing this ratio may result in a further improvement in the response, as discussed in more detail below. Depending on the size of the device, the size of the hole, the width of the collimator and other factors, a different minimum ratio can be developed.

Preferably, in a device 10 having a diameter of about 24 cm (8.5 inches), the ratio of the diameter of the device to the azimuthal width, or to the average azimuthal width, is less than about k%. In a preferred embodiment, most of the azimuthal width of the collimator for a device of this diameter is preferably about 0.6 cm (0.25 inch), so that the ratio is about 2.3%. If device 10 is not perfectly round, then the diameter referred to is the diameter of a circle defined by the outer edges of device 10.

This narrowing of the collimator 20 is to some extent contrary to the concept of widening the wedge of the collimator 20, as discussed, to accept a greater number of gamma rays scattered at greater angles, but this concept is mainly addressed by the limitation of the mean azimuthal width in the short-range collimator 20 and the preferred inclination of the short-range collimator towards the detector 14, as shown in Figures 1 and 3, and certainly applies to a significant portion of the short-range collimator 20.

The azimuthal acceptance angle and the length of the collimator become significant correction factors for measurement errors caused by the rotation of the device, compared to a wire device that does not rotate. The measurement errors caused by rotation can be demonstrated in carrying out a conventional calibration of the device.

A "static" rib, as we refer to the rib for the usual calibration of a device, can be determined by the response of the device 10, if the device is moved away from the borehole wall in such a way that the plane with the axis of the device, the source and the detector is perpendicular to the borehole wall, ie if the device is moved towards the diameter of the hole. In this situation, the distance, such as aperture 32, perceived by the short-range collier device is effectively symmetrical about a plane defined by the axis 34 of the device and the centers of the source and detectors.

In contrast, the distance or aperture 32 observed during rotation in too large a hole through the short-distance collimation device is usually not symmetrical about a plane defined by the axis 34 of the device and the centers from the source and detectors. This is because the shape of the distance or opening 32 (annulus) formed between the device 10 and the borehole wall 23 is different from the static case. In particular, the annulus will always be thicker on one side of the plane than on the other side, except in the two cases of symmetry during rotation, which occur when the device is at the minimum and maximum distances. We can designate a rib, determined by simply rotating an eccentrically rotating device in a hole that is too large, as "rotation rib".

The "static" and "rotation" ribs are not identical, ie the response of the short-range detector at a distance from the borehole wall 23 during eccentric rotation in too large a borehole differs from the response of the short- distance detector at a distance in too large a borehole, where the detector does not rotate. This is believed to be due to the asymmetry of the annular material observed by the short-range detector 14 and the non-linearity of the interaction between the gamma rays and the material involved. The latter makes any consideration of gamma ray interactions involving more than one material.

complicated.

Preferably, the manner in which this adverse situation is remedied, using the method and apparatus of the present invention, is by "narrowing" the short-range collation apparatus 14. As discussed, such "narrowing" preferably includes limiting the azimuthal width of the collimator and / or increasing the path length of the gamma rays in the collimator. These features effectively limit the width of the "beam" of gamma rays entering the detector. Since the effects of the asymmetry are observed across the width intersected by the radius of detected gamma rays, the narrower the radius, the smaller the effects of the asymmetry.

In other words, as the beam width is progressively reduced, the geometry of the device and the formation better approximates the static, symmetrical situation in which the core and rib characterization is usually performed. Thus, the response of the short-range detector to the distance from the borehole wall during eccentric rotation becomes approximately equal to the response of the short-range detector to the distance from the borehole wall during non-rotation of the detector, if the plane containing the source, detectors and axis of the device is perpendicular to the borehole wall. This also corresponds to the situation when rotating, if the distance is either maximum or minimum.

For the long-range detector 12, it is preferable that the radially oriented walls 46 and 48 of collimator 18 are each preferably held perpendicular to the longitudinal axis 34 of the device, in order to receive gamma rays from formation 18 from a promote radial direction with respect to the device 10, although they may also be slightly wedge-shaped, in order to increase the reception of gamma ray flux from the formation. Preferably, collimator 18 has no angle with respect to the radial direction, so that the flux of gamma rays from the formation is maximized. Edges 50 and 52 of the collimator, as shown in Fig. 6, may be perpendicular or, like the walls of the short-range collimator 20, may be wedge-shaped. Due to the increased distance between the detector 12 and the source 16, the effect of the decreased number of counts of gamma rays scattered at a wide angle is less obvious and preferably no correction is made. However, in order to increase the gamma flux received by the long range detector 12, since the gamma flux, as discussed above, has decreased, the mean azimuthal width of the long range has decreased. stand collimator, defined by edges 50 and 52, is at least three times the azimuthal width for the short range.

Borehole liner 30 is preferably a cylindrical sleeve and surrounds bore 38, which is passed through drilling fluid transfer device 10. Borehole liner 30 prevents radiation from source 16 from propagating through the drilling fluid 26 to bore 38 and to either the long-range detector 12 or the short-range detector 14, respectively, or blocks this radiation. Thus, borehole liner 30 preferably extends along the longitudinal length of at least a portion of the device 10 including the source and detectors.

Although the described collimators have substantially flat walls, the walls may also be round, oval, elliptical, etc., and may still have substantially the same or equivalent dimensions, according to the principles discussed. The foregoing description of the present invention is illustrative and illustrative thereof, and it will be apparent to those skilled in the art that various changes in size, shape and materials, as well as in details of the illustrated construction or combinations of features of the different elements may be made without to depart from the spirit of the invention.

Claims (91)

A method for determining at least one characteristic of an earth formation having a borehole therein, the borehole having a longitudinal axis, the method comprising the steps of: placing a first directional radiation sensor in the borehole next to the earthing formation, for detecting the counting pulses indicative of the radiation from the earth formation; rotating the first directional radiation sensor at a substantially fixed rotation speed, with a single rotation period; storing the counting pulses detected by the first directional radiation sensor during a total sampling period, the total sampling period being longer than twice the single rotation period, the step of storing the detected counting pulses storing the counting pulses during a a plurality of short periods within the total sampling period, each of the short periods being less than half of the single rotation period; defining a plurality of bins as a function of at least a portion of a distribution of the number of counts pulses detected during each short period; and sorting the count pulses detected during each short period into one of the plurality of bins to provide an indication representative of at least one characterization of the earth formation. The method of claim 1, wherein the step of defining a plurality of bins further comprises: averaging the number of count pulses detected during each short period; and defining the plurality of bins as a function of the determined average of the number of counts. The method of claim 2, further comprising: placing a second directional radiation sensor in the borehole at an axial distance from the first directional sensor; and placing a radiation source in the borehole at an axial distance from the first and second directional radiation sensors. l \. The method of claim 3. wherein the step of storing the counting pulses further comprises: storing the counting pulses detected by the second directed radiation sensor during a plurality of short periods; and adding up the counting pulses detected by the first and second directed sensors during the plurality of short periods. The method of claim 4, wherein the step of determining the average number of counting pulses further comprises: averaging the counting pulses from the first and second directed radiation sensors. The method of claim wherein the step of sorting the counting pulses further comprises: sorting the counting pulses detected during the short periods by the first and second radiation sensors into one or two bins, each defined as a function of the determined average of the number of counts. The method of claim 6, further comprising: weighting the counting pulses stored in one of at least three bins, to provide the indication representative of at least one characteristic of the earth formation. The method of claim 3. wherein the step of storing the counting pulses further comprises: passing the counting pulses for detection by the first and second directed radiation sensors during each of the short periods, which are primarily related to the energy levels that are associated with the Compton gamma ray scattering; and passing the counting pulses for detection by at least one of the first and second directional radiation sensors during each of the short periods, which are substantially related to the energy levels associated with the photoelectric absorption of gamma rays. The method of claim 3. further comprising: placing the second directional radiation sensor at a greater axial distance from the radiation source than the first directional radiation sensor; and blocking radiation received by the first directional radiation sensor from the formation in a direction substantially perpendicular to the longitudinal axis of the borehole. The method of claim 3. further comprising; transmitting radiation received by the second directional radiation sensor from the formation in a direction substantially perpendicular to the longitudinal axis of the borehole. The method of claim 3. further comprising: placing the first radiation sensor from the source at a selected distance to receive substantially only singly scattered gamma rays from the radiation source. The method of claim 2, further comprising: determining the theoretical standard deviation of the average number of counts and the measured standard deviation of the average number of counts; and comparing the theoretical standard deviation and the measured standard deviation, to produce a bore size indication signal. The method of claim 12, further comprising: correcting the stored count pulses in response to the borehole size indicator signal. A device for examining the properties of earth formations around a borehole, comprising a radiation source, a short-range detector and a long-range detector, each mounted in a body with a longitudinal axis, the device further comprising:> a short-range detector collimator disposed in the body in a prime location away from the radiation source, the short-range detector collimator defining a volume comprising material which is substantially transparent to radiation, the short-range detector collimator having a short-distance cross-section defined by a plane parallel to the longitudinal axis and comprising it, the far side of the short-distance cross-section, spaced from the radiation source, forms a first acute angle to the longitudinal axis for substantially blocking radiation received by the short-range detector or, from a direction perpendicular to the longitudinal axis of the device; and a long-range detector collimator disposed in the body in a second location away from the radiation source, the long-range detector collimator defining a volume containing a material which is substantially transparent for radiation, wherein the long-range detector collimating device has a wall extending outward from the long-range detector to radiate in a direction substantially perpendicular to the longitudinal axis of the device, to the long-range detector. The device of claim 14, wherein: the short-distance cross-section has a longitudinal length in a direction substantially parallel to the longitudinal axis, the longitudinal length increasing with the increased distance from the longitudinal axis in a radial outward direction. The device of claim 14, further comprising: a close-up side of the short-distance cross-section, which is located between the radiation source and the far side and which forms a second acute angle with respect to the longitudinal axis, which is sharper than the first sharp angle. The device of claim 11 further comprising: the short-range detector collimator of another cross section defined by another plane parallel to the longitudinal axis and spaced therefrom, the other cross section has a width in a direction perpendicular to the longitudinal axis, which decreases with increasing distance from the radiation source; and the long-range detector collimator with a substantially rectangular cross-sectional configuration in the other plane. The device of claim 14, further comprising: the body which is substantially tubular and includes a radially outwardly extending blade, the short-range detector collimator and the long-range detector collimator located each extending through a portion of the radially outwardly extending blade. The apparatus of claim 1, further comprising: the long range detector collimator in the other face, having a substantially constant width at a varying distance extending radially from the longitudinal axis; and the short-range collimator with a selected axial distance from the gamma ray source, for receiving substantially only singly scattered gamma rays from the radiation source. The device of claim 14, further comprising: a source collimating device made of a low density material and disposed in the body, the source collimating device having a cross section defined by a plane parallel to its length axis of the device and includes it, the cross section having a surface which increases radially outward from the longitudinal axis of the device. The device of claim 14, further comprising: the body having a central bore therethrough for passage of drilling fluid; and a sleeve made of a high-density material that is radially disposed between the well and the central borehole to prevent radiation from the well from contacting the drilling fluid in the central bore. The device of claim 21, further comprising: 3 the sleeve extending longitudinally from the radiation source to the long range detector. A method of determining at least one characteristic of an earth formation having a borehole therein, the borehole having a longitudinal axis, the method comprising the steps of: placing a radiation source in the borehole; placing a first directional radiation sensor in the borehole at some distance from the radiation source; placing a second directional radiation sensor in the borehole, relative to the first directional radiation sensor opposite the radiation source; and blocking the radiation received by the first directional radiation sensor from the earth formation in a direction substantially perpendicular to the longitudinal axis of the borehole. The method of claim 23, further comprising: transmitting radiation received by the second directional radiation sensor from the formation in a direction substantially perpendicular to the longitudinal axis of the borehole. The method of claim 24, further comprising: locating the first directional radiation sensor at a selected distance from the radiation source to receive substantially only singly scattered gamma rays from the radiation source. The method of claim 25. further comprising: forming a collimator configured to selectively transmit radiation received by the first directional radiation sensor from the radiation source such that the first directional radiation sensor scatters approximately the same number of times captures gamma rays that are scattered at a first angle compared to single-scattered gamma rays that are scattered at a second angle greater than the first angle. The method of claim 23, further comprising: rotating the first and two directional radiation sensors and the radiation source at a rotational speed with a single rotational period; detecting counts with the first and second directional radiation sensors during a measurement period comprising a plurality of short periods; and sorting the counting pulses detected during each of the plurality of short periods into one of a plurality of bins to provide an indication representative of at least one characteristic of the earth formation. The method of claim 27, further comprising: combining the counting pulses detected by the first directed radiation sensor and the second directed radiation sensor during the plurality of short periods. The method of claim 28, further comprising; averaging the counting pulses from the first and second directional radiation sensors. The method of claim 27, wherein the step of sorting the counting pulses further comprises: determining the average of the number of counting pulses for the counting pulses detected during the measurement period; and defining each of a plurality of bins as a function of the determined average of the number of counts. The method of claim 27, further comprising: weighting the counting pulses stored in a plurality of bins to provide the indication representative of at least one characteristic of the earth formation. The method of claim 27, wherein the step of sorting the counting pulses further comprises: passing the counting pulses for storage detected by each of the first and second directed radiation sensors during each of the short periods which are substantially related with the energy levels associated with the Compton scattering of gamma rays; and passing the storage counts detected by at least one of the first and second directional radiation sensors during each of the short periods, which are substantially related to the energy levels associated with the photoelectric absorption of gamma rays. A method according to claim 23. further comprising: placing an acoustic receiver in the borehole, next to the first and second directional radiation sensors, for generating an acoustic signal indicative of the scanning distance; correlating the acoustic signal with the counting pulses of the first and second directional radiation sensors, detected during a plurality of short periods, while rotating the acoustic receiver and the first and second directional radiation sensors; and sorting the counting pulses detected by the first and second directional sensors during each of a plurality of short periods, into a plurality of bins, each of which is functionally related to the acoustic signal. The method of claim 23. further comprising: placing an acoustic transceiver in the borehole, next to the first and second directional radiation sensors, to generate an acoustic signal indicative of the scan distance; correlating the acoustic signal with the counting pulses of the first and second directional radiation sensors, detected during a plurality of short periods, while rotating the acoustic receiver and the first and second directional radiation sensors; and determining the density of a formation in response to the counts of the first and second directional radiation sensors and the acoustic signal. A method according to claim 26, wherein the step of sorting the counting pulses comprises placing an acoustic sensor in the borehole to determine the distance; and the length of the measurement period is determined during the step of sorting the counting pulses. 36. A method of correcting measurements to determine at least one characteristic of an earth formation having a borehole therein, said method comprising the steps of: placing a first directional radiation sensor in the borehole next to the earthing formation to detect count pulses which are an indication of the radiation from the earth formation; rotating the first directional radiation sensor with a substantially fixed rotation speed with a single rotation period; detecting counts with the first directional radiation sensor during a total measuring period, the total measuring period being longer than twice the single rotation period, the step of detecting counts detecting counts with the first directional radiation sensor during a plurality of short periods, each within the total measurement period, each short period being less than half of the single rotation period; determining the measured standard deviation of the number of counting pulses from the distribution of the number of counting pulses; comparing the measured standard deviation of the number of counting pulses with the calculated standard deviation of the number of counting pulses; and producing an irregular borehole indication signal if the measured standard deviation of the number of counts by a maximum amount differs from the calculated standard deviation of the number of counts. The method of claim 36, further comprising: providing a correction to the detected counting pulses in response to the irregular indication signal. The method of claim 37, wherein providing a correction response to the irregular indication signal further comprises: sorting the count pulses during each of the short periods into one of a plurality of bins, each defined as a function of the distribution of the number of counts, to provide an indication representative of at least one characteristic of the earth formation. The method of claim 36, further comprising: determining the size of the borehole in response to the comparing step. The method of claim 39, further comprising: integrating a length of the borehole to determine the volume of the borehole. 41. A method of determining at least one characteristic of an earth formation having a borehole therein, said borehole containing drilling fluid, said method comprising the steps of: placing a radiation source in the borehole adjacent to the earth formation; placing a first directional radiation sensor in the borehole adjacent to the earth formation, for detecting counts indicative of radiation from the earth formation; placing a borehole acoustic sensor in alignment with the first directional radiation sensor all around to receive a plurality of acoustic signals, each indicative of the distance of the first directional radiation sensor from the earth formation in the borehole; rotating the source, the first directional radiation sensor and the borehole acoustic sensor at a rotation speed with a single rotation period; detecting counts with the first directional sensor during a total measurement period comprising a plurality of short periods; correlating each of the plurality of acoustic signals with the counting pulses detected during one of the plurality of short periods, respectively; defining a plurality of bins as a function of the plurality of acoustic signals; and sorting the counting pulses detected by the first directional sensor during each of the plurality of short periods in each of the respective bins. The method of claim 41, further comprising the step of: placing a second directional radiation sensor in the borehole, adjacent to the earth formation and aligned with the first directional radiation sensor, to detect count pulses indicative of the radiation from the earth formation. The method of claim 42, further comprising: determining the distance using the acoustic borehole scanner; and correcting the detected counts of the first and second directional radiation sensors as a function of the determined distance, to determine at least one characteristic of the borehole. A method according to claim 41, further comprising the steps of: determining the length of the total measurement period during the step of sorting the counting pulses. The method of claim 41, wherein the sorting step further comprises: defining each of a plurality of bins as a function of the travel time of the acoustic signal between the acoustic borehole sensor and the borehole wall. 46. A method of correcting measurements to determine at least one characteristic of an earth formation having a borehole therein defined by a wall, said method comprising the steps of: placing a well in the borehole about the borehole and the earthing formation irradiate with gamma rays; placing a first remote radiation sensor in the borehole, next to the earth formation, to detect count pulses indicative of the radiation from the earth formation; placing a second remote radiation sensor in the borehole at a distance from the source greater than the distance from the first remote radiation sensor; placing a housing for the device in the borehole, to provide a support for the well, the first remote radiation sensor and the second remote radiation sensor, the housing for the device having a diameter which is related is on the circle around the case; forming a first collimator configured to pass a portion of the gamma rays to the first remote radiation sensor, the azimuthal width of a portion of the collimator being less than about k% of the diameter of the housing of the establishment; rotating the source and the first and second remote radiation sensors; and detecting gamma rays with the first remote radiation sensor around at least a substantial portion of the borehole circumference during the rotation step. The method of claim 46, further comprising the step of: forming a second collimator for the second remote radiation sensor having an azimuthal width greater than the azimuthal width of the portion of the first collimator. The method of claim 46, wherein: the first collimator has an azimuthal width that varies along its length and provides an average azimuthal width, the average azimuthal width of the first collimator being less than the azimuthal width of the second collimator. The method of claim 46, further comprising the step of: adjusting the azimuthal width of the portion of the collimator to the rotational and non-rotational response of the first remote radiation sensor to the distance from the collimator. wall of the borehole are almost the same. 50. A method of correcting measurements to determine at least one characteristic of an earth formation containing a borehole defined for a wall, said method comprising the steps of: placing a well in the borehole about the borehole and the earth formation irradiate with gamma rays; placing a first remote radiation sensor in the borehole, next to the earth formation, to detect count pulses indicative of the radiation from the earth formation; placing a second remote radiation sensor in the borehole at a greater distance from the source than the first remote radiation sensor; forming a first collimator configured to transmit a portion of the gamma rays to the first remote radiation sensor, the first collimator having a portion having a first azimuthal width; and forming a second collimator configured to transmit a portion of the gamma rays to the second remote radiation sensor, the second collimator having a portion having a second azimuthal width greater than at least three times the first azimuthal width. The method of claim 50, further comprising: placing a housing for the device in the wellbore, to provide a support for the well, the first remote radiation sensor and the second remote radiation sensor, wherein the housing for the device has a diameter related to the circle around the housing; forming a first collimator such that the azimuthal width of a portion of the collimator is less than about 4% of the diameter of the housing of the device; rotating the source and the first and second remote radiation sensors; and detecting gamma rays with the first remote radiation sensor around at least a substantial portion of the borehole circumference during the rotation step. The method of claim 50, further comprising: extending a length through the first collimator by tilting the first collimator at a distance from the radial direction relative to the first remote detector; and providing a first azimuthal width such that the ratio of the length to the first azimuthal width is greater than about six. Modified conclusions [received on May 30, 1995 (May 30, 1995) by the International Bureau; originally claims 15. 21-35 and 41-45 lapsed; original claims 1. 7. 14, 17, 19. 36, 38, 46, 48, 49 and 52 modified; new conclusions 53_64 added (12 pages)] A method for determining at least one characteristic of an earth formation having a borehole therein, the borehole having a longitudinal axis, the method comprising the steps of: placing a first directional radiation sensor in the borehole next to the earthing formation, for detected counts that are indicative of radiation from the earth formation; rotating the first directional radiation sensor at a substantially fixed rotation speed, with a single rotation period; storing the counting pulses detected by the first directional radiation sensor during a total measuring period, the total measuring period being longer than twice the single rotation period, the step of storing the detected counting pulses storing the counting pulses during a plurality of includes short periods within the total measurement period, each of the short periods being less than half of the single rotation period; defining a plurality of bins as a function of at least a portion of a distribution of the number of counts pulses detected during each short period; and sorting the counting pulses detected during each short period into one of the plurality of bins to provide an indication representative of at least one characteristic of the earth formation. The method of claim 1, wherein the step of defining a plurality of bins further comprises: averaging the number of count pulses detected during each short period; and defining the plurality of bins as a function of the determined average of the number of counts. The method of claim 2, further comprising: placing a second directional radiation sensor in the borehole at an axial distance from the first directional sensor; and placing a radiation source in the borehole at an axial distance from the first and second directional radiation sensors. The method of claim 3. wherein the step of storing the counting pulses further comprises: storing the counting pulses detected by the second directed radiation sensor during a plurality of short periods; and adding up the counting pulses detected by the first and second directed sensors during the plurality of short periods. The method of claim 4, wherein the step of determining the average number of counting pulses further comprises: averaging the counting pulses from the first and second directed radiation sensors. The method of claim 4, wherein the step of sorting the counting pulses further comprises: sorting the counting pulses detected by the first and second radiation sensors during the short periods into one or two bins, each of which are defined as a function of the determined average of the number of counts. The method of claim 6, further comprising: weighting the counting pulses stored in one of at least two or more bins, to provide the indication representative of at least one characteristic of the earth formation. > The method of claim 3. wherein the step of storing the counting pulses further comprises: passing the counting pulses for detection by the first and second directed radiation sensors during each of the short periods, which are substantially related to the energy levels associated with Compton gamma ray scattering; and passing the counting pulses for detection by at least one of the first and second directional radiation sensors during each of the short periods, which are primarily related to the energy levels associated with the photoelectric absorption of gamma rays. The method of claim 3, further comprising: placing the second directional radiation sensor at a greater axial distance from the radiation source than the first directional radiation sensor; and blocking radiation from the formation, from a direction substantially perpendicular to the longitudinal axis of the borehole received by the first directional radiation sensor. The method of claim 3. further comprising; transmitting radiation received by the second directional radiation sensor from the formation in a direction substantially perpendicular to the longitudinal axis of the borehole. The method of claim 3. further comprising: placing the first radiation sensor from the source at a selected distance to receive substantially only singly scattered gamma rays from the radiation source. The method of claim 2, further comprising: determining the theoretical standard deviation of the average number of counts and the measured standard deviation of the average number of counts; and comparing the theoretical standard deviation and the measured standard deviation, to produce a bore size indication signal. The method of claim 12, further comprising: correcting the stored count pulses in response to the borehole size indicator signal. A device for examining the properties of earth formations around a borehole, comprising a radiation source, a short-range detector and a long-range detector, each mounted in a body with a longitudinal axis, the device further comprising: a short-range detector collimator disposed in the body in a primary location remote from the radiation source, the short-range detector collimator defining a volume comprising material that is substantially transparent to radiation the short-range detector collimator having a short-distance cross-section defined by a plane parallel to the longitudinal axis and comprising it, the far side of the short-distance cross-section being is spaced from the radiation source, forms a first acute angle with respect to the longitudinal axis, to substantially block radiation received by the short-range detector, u it is a direction perpendicular to the longitudinal axis of the device, the short-distance cross-section having a longitudinal length in a direction substantially parallel to the longitudinal axis, the longitudinal length increasing with increasing distance of the longitudinal axis, in a radially outward direction; and a long-range detector collimator disposed in the body in a second location, away from the radiation source, the long-range detector collimator defining a volume containing a material that is substantially transparent for radiation, wherein the long-range detector collimating device has a wall extending outward from the long-range detector to direct radiation in a direction substantially perpendicular to the longitudinal axis of the device, to the long-range detector. 15. (Expired) The device of claim 1 * 1, further comprising: a close-up side of the short-distance cross-section, which is located between the radiation source and the far side and which forms a second acute angle with respect to the longitudinal axis, which sharper than the first sharp angle. The apparatus of claim 1 * 1, further comprising: the short range detector collimator of another cross section defined by another plane that is parallel to the longitudinal axis and spaced therefrom, the other cross-section having a width in a direction perpendicular to the longitudinal axis, which increases with an increasing distance from the radiation source; and the long-range detector collimator with a substantially rectangular cross-sectional configuration in the other plane. The device of claim 14, further comprising: the body which is substantially tubular and includes a radially outwardly extending blade, the short-range detector collimator and the long-range detector collimator located each extending through a portion of the radially outwardly extending blade. The device of claim 14, further comprising: the substantially constant width long range detector collimator having a varying distance extending radially from the longitudinal axis; and the short-range collimator with a selected axial distance from the radiation source, for receiving substantially only singly scattered gamma rays from the radiation source. The device of claims 1-4, further comprising: a source collimator made of a low density material and disposed in the body, the source collimator having a cross section defined by a plane parallel to the longitudinal axis of the device and includes the cross-section having a surface which increases radially outward from the longitudinal axis of the device. 21. -35 · (Expired) 36. A method of correcting measurements to determine at least one characteristic of an earth formation having a borehole therein, said method comprising the steps of: placing a first directional radiation sensor in the borehole next to the earthing formation to detect count pulses which are an indication of the radiation from the earth formation; rotating the first directional radiation sensor at a substantially fixed rotation speed with a single rotation period; detecting counts with the first directional radiation sensor during a total measuring period, the total measuring period being longer than twice the single rotation period, the step of detecting counts detecting counts with the first directional radiation sensor during a plurality of short periods, each within the total measurement period, each short period being less than half of the single rotation period; determining the measured standard deviation of the number of counting pulses from the distribution of the number of counting pulses detected during the plurality of short periods; comparing the measured standard deviation of the number of counting pulses with the calculated standard deviation of the number of counting pulses; and producing an irregular borehole indication signal if the measured standard deviation of the number of counts by a maximum amount differs from the calculated standard deviation of the number of counts. The method of claim 36, further comprising: providing a correction to the detected counting pulses in response to the irregular indication signal. The method of claim 37, wherein the step of providing a correction response to the irregular indication signal further comprises: sorting the count pulses during each of the short periods into one of a plurality of bins, each defined as a function of the distribution of the number of counts, to provide an indication representative of at least one characteristic of the earth formation. The method of claim 36, further comprising: determining the size of the borehole in response to the comparing step. * 40. The method of claim 39, further comprising: integrating a distance from the borehole to determine the volume of the borehole. 41.-45- (Repealed) 46. A method of correcting measurements to determine at least one characteristic of an earth formation having a borehole therein defined by a wall, said method comprising the steps of: placing a well in the borehole about the borehole and the earthing formation irradiate with gamma rays; placing a first remote radiation sensor in the borehole, next to the earth formation, to detect count pulses indicative of the radiation from the earth formation; placing a second remote radiation sensor in the borehole at a distance from the source greater than the distance from the first remote radiation sensor; placing a housing for the device in the borehole, to provide a support for the well, the first remote radiation sensor and the second remote radiation sensor, the housing for the device having a diameter which is related is on the circle around the case; forming a first collimator configured to pass a portion of the gamma rays to the first remote radiation sensor, the azimuthal width of a portion of the collimator being less than about 4% of the diameter of the housing of the establishment; rotating the source and the first and second remote radiation sensors; and detecting gamma rays with the first remote radiation sensor around at least a substantial portion of the borehole circumference during the rotation step. The method of claim 46, further comprising the step of: forming a second collimator for the second remote radiation sensor having an azimuthal width greater than the azimuthal width of the portion of the first collimator. The method of claim 47. wherein: the first collimator has an azimuthal width that varies along its length and gives an average azimuthal width, the average azimuthal width of the first collimator being less than the azimuthal width of the second collimator. The method of claim 46, further comprising the step of: adjusting the azimuthal width of the portion of the collimator to the rotational and non-rotational response of the first remote radiation sensor to the distance from the collimator. wall of the borehole are almost the same. 50. A method of correcting measurements to determine at least one characteristic of an earth formation containing a borehole defined for a wall, said method comprising the steps of: placing a well in the borehole about the borehole and the earth formation irradiate with gamma rays; placing a first remote radiation sensor in the borehole, next to the earth formation, for detecting counts indicative of radiation from the earth formation, placing a second, remote radiation sensor in the borehole, greater distance from the source than the first remote radiation sensor; forming a first collimator configured to pass a portion of the gamma rays to the first remote radiation sensor, the first collimator having a portion having a first azimuthal width, and forming a second collimator is configured to transmit a portion of the gamma rays to the second remote radiation sensor, the second collimator having a portion having a second azimuthal width greater than at least three times the first azimuthal width. The method of claim 50, further comprising: placing a housing for the device in the wellbore, to provide a support for the well, the first remote radiation sensor and the second remote radiation sensor, wherein the housing for the device has a diameter related to the circle around the housing; forming a first collimator such that the azimuthal width of a portion of the collimator is less than about the diameter of the housing of the device; rotating the source and the first and second remote radiation sensors; and detecting gamma rays with the first remote radiation sensor around at least a substantial portion of the borehole circumference during the rotation step. The method of claim 50, further comprising: tilting the first collimator with respect to a radial line perpendicular to the longitudinal axis of the housing of the apparatus for increasing the axial length through the first collimator; and providing a first azimuthal width such that the ratio of the axial length to the first azimuthal width is greater than about six. 53. Apparatus for examining the properties of earth formations around a borehole, comprising a radiation source, a short-range detector and a long-range detector, each mounted in a body with a longitudinal axis, which device further comprises: the body with a central bore therethrough for the passage of drilling fluid; a sleeve of high density material radially disposed between the well and the center bore to prevent radiation from the source from contacting the drilling fluid in the center bore; a short-range detector collimator, which is disposed in the body at a distance from the radiation source, the close-detector collimator defining a volume containing material that is substantially transparent to radiation, the short range detector collimator has a short distance cross section defined by and includes a plane parallel to the longitudinal axis; and a long-range detector collimator disposed in the body in a second location, away from the radiation source, the long-range detector collimator defining a volume containing a material containing substantially is transparent to radiation. The device of claim 53, wherein the sleeve extends longitudinally from the radiation source to the long-range detector. The device of claim 53, further comprising: a source collimator made of a low density material and disposed in the body, the source collimator having a cross section defined by a plane parallel to the longitudinal axis of the device and comprising the cross-section having a surface which increases radially outward from the longitudinal axis. 56. A method of determining at least one characteristic of an earth formation having a borehole therein, said borehole having a longitudinal axis, said method comprising the steps of: placing a radiation source in the borehole; placing a first directional radiation sensor in the borehole at a location remote from the radiation source; placing a second directional radiation sensor in the borehole with respect to the first directional radiation sensor opposite the radiation source; rotating the first and second directional radiation sensors and the radiation source at a rotation speed with a single rotation period; detecting counts with the first and second directional radiation sensors during a measurement period comprising a plurality of short periods; and sorting counts detected during each of the plurality of short periods into one of a plurality of bins to provide an indication representative of at least one characteristic of the earth formation. The method of claim 56, further comprising: combining the counting pulses detected by the first directed radiation sensor and by the second directed radiation sensor during that plurality of short periods. The method of claim 57, further comprising: averaging the count pulses of the first and second directed radiation sensors. The method of claim 56, wherein the step of sorting the counting pulses further comprises: averaging the number of counting pulses detected during the measurement period; and defining each of a plurality of bins as a function of the determined average number of counts. The method of claim 56, further comprising: weighting counting pulses stored in one of a plurality of bins to provide the indication representative of at least one characteristic of the earth formation. The method of claim 56, wherein the step of sorting the counting pulses further comprises: passing counting pulses for storage detected by the first and second directional radiation sensors during each of the short periods, which are significantly related to energy levels associated with Compton gamma ray scattering; and passing storage counts detected by at least the first or second directional radiation sensors during each of the short periods, which are significantly related to energy levels associated with the photoelectric absorption of gamma rays. The method of claim 56, further comprising: the step of sorting detected counting pulses comprises placing an acoustic sensor in the borehole to determine the distance; and determining the length of the measurement period during the step of sorting the detected counting pulses. 63. A method of determining at least one characteristic of an earth formation having a borehole therein, said borehole having a longitudinal axis, said method comprising the steps of: placing a radiation source in the borehole; placing a first directional radiation sensor in the borehole at a location remote from the radiation source; placing a second directional radiation sensor in the borehole, relative to the first directional radiation sensor opposite the radiation source; placing a receiver in the borehole, next to the first and second directional radiation sensors, to generate a signal indicative of the distance; correlating the signal with the counting pulses from the first and second directional radiation sensors detected during a plurality of short periods while the receiver and the first and second directional radiation sensors are rotated; and sorting the counting pulses detected during each of the plurality of short periods by the first and second directional sensors into a plurality of bins, each of which is functionally related to the signal. 6 ^. A method according to claim 63, further comprising: the step of placing the receiver comprises placing an acoustic transmitter-receiver in the borehole, next to the first and second directional radiation sensors, to generate an acoustic signal indicative for the distance; correlating the acoustic signal with the counting pulses of the first and second directional radiation sensors detected during a plurality of short periods while the acoustic receiver and the first and second directional radiation sensors are rotated; and determining the density of a formation in response to the counts of the first and second directional radiation sensors and the acoustic signal.